Stem cells are primal cells that retain the ability to renew themselves through cell division and can differentiate into a wide range of specialized cell types. Research in the stem cell field grew out of findings by Canadian scientists Ernest A. McCulloch and James E. Till in the 1960s.
The two categories of stem cells include embryonic stem cells and adult stem cells and possibly a third, cord blood-derived embryonic-like stem cells (CBEs). In a blastocyst of a developing embryo, stem cells differentiate into all of the specialized embryonic tissues. In adult organisms, stem cells and progenitor cells act as a repair system for the body, replenishing specialized cells. This makes stem cells interesting for therapeutic applications.
Totipotent stem cells are produced from the fusion of an egg and sperm cell. Cells produced by the first few divisions of the fertilized egg cell are also totipotent. These cells can differentiate into embryonic and extraembryonic cell types.
Pluripotent stem cells are the descendants of totipotent cells and can differentiate into cells derived from the three germ layers.
Multipotent stem cells can produce only cells of a closely related family of cells (e.g. hematopoietic stem cells differentiate into red blood cells, white blood cells, platelets, etc.).
Unipotent cells can produce only one cell type, but have the property of self-renewal which distinguishes them from non-stem cells.
The term progenitor cell is used in cell biology and developmental biology to refer to immature or undifferentiated cells. While progenitor cells share many common features with stem cells, the term is far less restrictive. The majority of progenitor cells lies dormant or possesses little activity in the tissue in which they reside. They exhibit slow growth and their main role is to replace cells lost by normal attrition.
Embryonic stem cells (ES cells) are stem cells derived from the inner cell mass of a blastocyst. ES cells are pluripotent, and give rise during development to all derivatives of the three primary germ layers: ectoderm, endoderm and mesoderm. In other words, they can develop into each of the more than 200 cell types of the adult body when given sufficient and necessary stimulation for a specific cell type. When given no stimuli for differentiation, ES cells will continue to divide in vitro and each daughter cell will remain pluripotent. The pluripotency of ES cells has been rigorously demonstrated in vitro and in vivo, thus they can be indeed classified as stem cells. Because of their unique combined abilities of unlimited expansion and pluripotency, embryonic stem cells are a potential source for regenerative medicine and tissue replacement after injury or disease. To date, no approved medical treatments have been derived from embryonic stem cell research,
ES cells were first derived from mouse embryos in 1981 by two independent research groups (Evans & Kaufman and Martin). A breakthrough in human embryonic stem cell research came in November 1998 when a group led by James Thomson at the University of Wisconsin-Madison first developed a technique to isolate and grow the cells when derived from human blastocysts.
Adult stem cells reside in nearly every tissue, including the brain, bone marrow, peripheral blood, kidney, epithelia of the digestive system, and also the skin, retina, muscles, pancreas and liver (Blau et al., 2001; Tarnowski and Sieron, 2006). They contribute to tissue homeostasis and regeneration after damage. However, their specific properties are often elusive because of their heterogeneity and technical difficulties in identifying these rare cells and their progeny. Hematopoietic stem cells are the best characterized adult stem cells. They are able to reconstitute all cells of the blood. Stem cells also exist in the adult brain (Gage, 2000). These so-called neural stem cells give rise to neurons and glial cells in tissue culture or following direct injection into brains. Neural stem cells can be obtained from adult brain tissue following proteolytic dissociation and density gradient centrifugation. They can be grown and expanded as ‘neurospheres’ in culture (Quesenberry et al., 1999). Mesenchymal (stromal) stem cells derived from bone marrow, umbilical cord, placenta or adipose tissue give rise to adipocytes, chondrocytes, tenocytes and osteocytes. However, they also possess a remarkable degree of plasticity when exposed to specific environmental factors (Grove at al., 2004). Multipotent adult progenitor cells are pluripotent cells obtained in culture from mesenchymal stem cells (Jiang of al., 2002). Recently, pluripotent spermatogonial stem cells were isolated from adult mouse testis (Gunn et al., 2006).
Pluripotent cells can also be obtained from somatic cells by nuclear reprogramming, including cloning, cell fusion and cytoplasmic mixing (Pomerantz and Blau, 2004).
The goal of regenerative medicine is to restore tissue function. Stem cells and progenitor cells offer the potential to replace lost or damaged cells and to create a milieu supportive of functional recovery. Clinical interventions can encompass stimulation of endogenous stem and progenitor cells populations. Unfortunately, most of the newly generated cells do not fully differentiate or die or fail to migrate to the sites of damage. These obstacles are exemplified in a study on neuronal replacement from endogenous neural precursors after cerebral ischemia (Arvidsson et al., 2002). Erythropoietin was recently shown to enhance and/or induce the migration of multipotent neural stem cells and their progeny (see patent WO 2004/011021). Administration of exogenous stem or progenitor cells is an equally appealing avenue for the treatment of diseases which are refractory to most other treatments. ES and ES-like cells would seem ideally suited for stem cell therapy due to their pluripotency and self-renewal capacity. However, the use of ES cells is ethically controversial. Moreover, challenges to overcome are making the stem cells differentiate into specific viable cells consistently, and controlling against unchecked cell division because undifferentiated embryonic stem cells can form teratomas after transplantation. Allogeneic transplantation of stem cells and progenitor cells also carries the likelihood of immune rejection. Transplantation of adult stem cells may overcome some of these obstacles, but the differentiative potential of these cells is limited and many cells die after transplantation. Thus, pre-differentiation of the stem cells ex vivo may be required for their functional integration into target tissue. Adult stem cells are difficult to expand or even to maintain in culture (e.g. expansion of naive hematopoietic stem cells is impossible ex vivo). In general, it is laborious to keep stem and progenitor cells growing, well-nourished and stable in the laboratory so they do not die or turn into a cell type with less potential. Further, purification of the stem or progenitor cells is required prior to transplantation because a mixed population of cells could cause the growth of unwanted tissues. The purification procedure is very strenuous on the cells and often associated with substantial cell loss.
The invention of methods of cell culture and treatment comprising a vEPO protein variant will help to overcome many of the difficulties associated with stem or progenitor cell therapy.